This invention relates to gear drives, and particularly to the air cooling of gear drives by use of an enclosure for the gear drive housing through which air is passed to cool the drive.
The mechanical rating of large enclosed gear drives of a given housing size has been increasing significantly because of stronger and harder gear materials, more accurate gear cutting and finishing methods, advanced bearing designs, and improved lubricants. Expressed in terms of mechanical load intensity, which is the mechanical rating per square foot of housing surface area, present gear drives have two to three times the load intensity of those of only two decades ago. This significantly greater load intensity has taxed the ability of the gear drives to transmit actual service horsepower equal to the mechanical rating without overheating. Overheating will manifest itself in unacceptably high temperatures in the lubricant sump in the base of the drive housing.
A number of different approaches have been proposed for cooling large enclosed gear drives. The approaches generally involve mechanical air cooling and water cooling.
In the simplest form, mechanical air cooling involves directing a fan against the exterior of the gear drive enclosure. Typically, one or more fans have been mounted on the shafts of the gear drive, usually on the input shaft because it operates at the highest speed. The addition of a shaft mounted fan or fans adds forced convection heat transfer and can increase the heat dissipation capacity significantly. However, the effectiveness of the shaft mounted fans depends on the fan diameter and input shaft speed. Shaft mounted fans often do not provide enough cooling since the size of the fan is limited by the physical size of the drive and the speed is limited to that of the application for the gear drive.
A separate electric fan can be positioned close to the gear drive housing and with or without a baffle to direct the air along the sides and top of the housing. One such arrangement is shown in U.S. Pat. No. 3,406,591 issued Oct. 22, 1968 to Homiller. This approach avoids the speed and size limitations of the shaft mounted fans but still provides insufficient cooling in many applications.
Sufficient air cooling has only been obtained heretofore with an external oil-to-air heat exchanger. A pump circulates the sump lubricating oil through a radiator that is cooled by an electric fan. This approach requires extensive piping and other external equipment.
Traditionally, water cooling has used an external oil-to-water heat exchanger with a pump to circulate the lubricating oil from the housing sump. For a large enclosed gear drive the flow rates of both the oil and water through the external cooler are very substantial, ranging up to 25 gallons per minute.
An alternative to such an external cooling method for the oil is the approach shown in U.S. Pat. No. 4,633,938 issued Jan. 6, 1987 to Schunck, et al., which employs internal, finned cooling tubes in the base of the drive housing.
Still another water cooling method involves cooling the air in the housing by use of an external heat exchanger and a pump for circulating the air from the housing through the exchanger and back to the housing. This later approach is shown in U.S. Pat. No. 3,736,812 issued June 5, 1973 to Wellauer.
Although water cooling may be the most effective, water may not be available in particular installations. Also, water cooling generally adds substantial piping and other external equipment to the gear drive installation.